Rotary processors have been used to advantage for processing plastic and polymeric materials. Details and features relating to such processors are described in U.S. Pat. Nos. 4,142,805; 4,411,532 and the references cited in the latter. Complications have arisen in applications when a rotary processor is used in a process where it is subject to upstream fluctuations in mass flow or pressure, becuase of disturbances or process instabilities occurring upstream of the rotary processor or in upstream stages within the rotary processor itself or due to melting rate variations. Since a rotary processor is an efficient conveyor of viscous materials, such fluctuations are translated or transmitted downstream by the flow of the material and may adversely affect process control within the rotary processor and thus undesirably affect product quality, throughput, or discharge rates.